Catalytic converters are used in vehicular exhaust systems to convert certain objectionable gases into environmentally more acceptable forms. The catalyst in the converter becomes operative or "lights-off" after it is heated to a specified temperature by the exhaust gases. Prior to lighting-off, the catalytic converter falls considerably short of complying with the air quality standards in most jurisdictions.
The walls of the exhaust pipe leading from the engine to the catalytic converter are cold when the vehicle is first started. These cold pipes function as heat sinks which effectively draw heat from the exhaust gases traveling therethrough, and hence retard the lighting-off of the catalytic converter. The amount of heat dissipated in the walls of the pipe between the engine and the catalytic converter is a function of the thermal mass of the pipe, which in turn is a function of the pipe length, pipe diameter, surface area and pipe thickness.
Catalytic converters typically are disposed as close as possible to the upstream end of an exhaust system where the exhaust gas is hottest. The upstream location is intended to accelerate the heating of the catalyst, and thereby achieves a quicker light-off. However, the engine compartment of a vehicle generally is too crowded to accommodate a catalytic converter and the required heat shields. Hence, the converter normally is disposed downstream of the engine compartment. Crowding in the engine compartment also requires a circuitous routing of the exhaust pipe extending from the engine manifold, through the engine compartment and to the catalytic converter. The circuitous routing of the exhaust pipe leading to the catalytic converter adds to the total length of the exhaust pipe. Thus, there is often a relatively long run of exhaust pipe between the manifold and the catalytic converter, even in instances where the catalytic converter can be placed fairly close to the engine compartment.
Thin pipes draw less heat from exhaust gas than thicker pipes in view of the lower thermal mass of the thinner pipes. However, pipes in the engine compartment are subject to almost continuous vibration and frequent shock. Thin pipes are likely to behave poorly in response to vibration and shock. Additionally, normal engine maintenance requires workers to periodically access the engine compartment with tools. Forcible contact with a large tool can deform or otherwise damage a thin-walled pipe in the engine compartment. A deformation can have a significant effect on the flowing exhaust gas. Exhaust-pipes undergo frequent thermal expansion and contraction which can exert significant stresses and strains on the thin pipe. Thus, automotive engineers are faced with competing demands of having a structurally adequate exhaust pipe and one that will enable the catalytic converter to light-off as quickly as possible.
Heated components of an exhaust system often must be shielded at certain locations. For example, shields are used under exhaust system components that are close to the ground to avoid creating fires in nearby leaves or grass. Similarly, shields often are disposed between the exhaust system and parts of the vehicle that are sensitive to high temperatures.
Many prior art heat shields are stamped from sheets of metal and are held in proximity to the exhaust system component by straps or welding. The prior art also includes air gap pipes to protectively separate the heated exhaust system component from adjacent parts of the vehicle or from combustible materials on the ground. Air gap pipes are relatively easy to manufacture for straight sections of an exhaust pipe. However, many prior art air gap pipe designs are difficult and costly to manufacture for circuitously aligned pipe sections. One costly approach for making a bent air gap pipe requires a linear inner pipe to be supported within a linear outer pipe by a filler material that has a lower melting point than either of the pipes. The linear assembly of inner and outer pipes and filler material is then bent into the required circuitous shape. The bent assembly is then heated sufficiently for the filler material to melt and be poured from the generally annular space between the inner and outer pipes. This approach is effective, but very costly and time consuming.
Another very effective air gap pipe and method of manufacture is shown in U.S. Pat. No. 4,501,302 and in U.S. Pat. No. 4,656,713 both of which are assigned to the assignee of the subject invention. The air gap pipes shown in these two prior patents are made by initially bending the inner and outer pipes into the required shape. The outer pipe is then cut longitudinally in half and is sandwiched around the comparably bent inner pipe.
Prior art air gap pipes perform their intended function as heat shields, but do not overcome the above described problems of the circuitous pipe in the engine compartment of a vehicle functioning as a heat sink which yields undesirable delays in lighting-off of a catalytic converter.
In view of the above, it is an object of the subject invention to provide an exhaust pipe assembly enabling a quicker light-off of a catalytic converter.
It is another object of the subject invention to provide a low thermal mass exhaust pipe exhibiting adequate structural integrity.
A further object of the subject invention is to provide a catalytic converter assembly with an ability to light-off quickly.
Still another object of the subject invention is to provide a method for manufacturing an exhaust pipe assembly for a catalytic converter.
An additional object of the subject invention is to provide an air gap pipe assembly with low heat transfer from the inner pipe to the outer pipe.
Yet another object of the subject invention is to provide an air gap pipe assembly that substantially avoids excessive stresses and strains in response to thermal expansion and contraction.